In general, the cargo space in commercial aircraft is inaccessible during flight. For this reason, most commercial aircraft rely on automatic fire-extinguishing systems to extinguish any fire which occurs in the cargo space and to keep the fire suppressed for the duration of the flight.
The original approach to protecting the cargo space was to have two or more agent containers, each having sufficient agent to produce a concentration greatly in excess of that required for extinguishing a fire in the cargo space. These containers, which were rapid discharge containers, would be discharged sequentially at predetermined time intervals. For example, the first container would be discharged at the onset of a fire to raise the concentration of fire-extinguishing material in the cargo space to about 8%-10%. This concentration would decay due to leakage of fire-extinguishing material from the cargo space. Before the concentration dropped below the critical value needed t keep the fire suppressed, a second container would be fired, again producing a concentration of 8%-10%. This method of keeping a fire suppressed proved inefficient in that it requires more fire-extinguishing material than is necessary to keep the fire suppressed.
Today, most fire extinguishing systems include at least two containers --one for rapid discharge of the fire extinguishing material and one for extended discharge of the fire extinguishing material. Both containers are normally disposed outside the cargo space and are subject to temperatures between -65.degree. F. and 170.degree. F. The rapid discharge container is designed to rapidly release the fire-extinguishing material so as to obtain, in a short time period, a concentration in the cargo space sufficient to extinguish or "knock-down" the fire. Because the fire-extinguishing material will gradually leak from the cargo space, the extended discharge container is designed to release the fire-extinguishing material at a much slower rate in order to keep the concentration of fire-extinguishing material in the cargo space at a predetermined value to prevent the fire from reoccurring. Without the extended discharge, the concentration of fire-extinguishing material in the cargo space could drop below what is necessary to keep the fire suppressed and embers could reignite the fire.
In prior art systems, little concern is given to maintaining a predetermined mass flow rate of the fire-extinguishing material into the cargo space. For example, some extended discharge systems simply discharge the fire extinguishing material through a fixed orifice. Testing of one such system has shown that the mass flow rate of fire-extinguishing material may vary by 38% depending on the temperature of the fire-extinguishing material. Another system uses a constant pressure regulator in series with a fixed orifice. Tests of this system revealed variations in mass flow rate of more than 600%.
Variations in the mass flow rate of fire-extinguishing material in an extended discharge system is potentially hazardous. If the mass flow rate is too low to maintain an adequate concentration of fire-extinguishing material in the cargo space the fire could reoccur. Conversely, if the mass flow rate is too high, then the fire-extinguishing material could be depleted before the flight is completed. This possibility would require the extended discharge container to be grossly over-sized to provide an adequate safety margin resulting in higher costs and greater weight to be carried.
Accordingly, there is a great need for an extended discharge system which can produce a generally constant mass flow rate over a wide temperature range.